Polymer gels consisting of cross-linked polymer networks immersed in a solvent are known to undergo reversible volume phase transitions upon small changes in the environment. See Tanaka, T., et al, Science 218, 467-469 (1982) and Okajima T., et al, J. of Chem. Phys. 20 (116), 9068-9077 (2002). However, this property has not heretofore been used to move the center of mass of the gel.